1. Field of the Invention
The present invention relates to a charging circuit which charges a secondary battery based on an external power supply voltage, particularly to an overvoltage protection technique of the charging circuit. The present invention also relates to an overvoltage protection circuit which protects a circuit from an overvoltage.
2. Description of the Related Art
Recently, many electronic circuits such as CPU (Central Processing Unit) which performs digital signal processing, DSP (Digital Signal Processor), a liquid crystal panel, or other analog and digital circuits are mounted on various electronic equipments such as a portable phone, PDA (Personal Digital Assistant), and a notebook personal computer. In a battery-driven electronic equipment on which a battery is mounted as a power supply, each electronic circuit in the equipment is operated by a battery voltage from the battery.
In the case where the battery is a secondary battery such as a lithium-ion battery, a charging circuit is incorporated into the electronic equipment. The charging circuit receives a voltage supplied from an external AC adaptor or the like, and the charging circuit supplies a charging current to the battery. For example, Japanese patent Application Laid-Open No. 9-219935 discloses the related technique.
There is a breakdown voltage in a circuit element such as a resistor and a transistor, which constitutes the electronic circuit. When the voltage exceeding the breakdown voltage is applied, the electronic circuit cannot perform the normal function.
[Patent Document] Japanese patent Application Laid-Open No. 9-219935
(1) The voltage given to the charging circuit from the outside is not always kept constant. For example, sometimes the extremely high voltage is supplied from the AC adaptor incompatible with the electronic equipment or other power supply circuits. When the voltage supplied from the outside exceeds the breakdown voltage of the charging circuit, not only a charging operation is affected, but also reliability is possibly affected in the charging circuit, the battery, and the electronic equipment.
(2) Generally, because CPU and DSP are operated by the voltage which is stabilized by the power supply circuit incorporated into the electronic equipment, the unexpected overvoltage is rarely applied to CPU and DSP. On the other hand, the charging circuit which charges the battery receives the power supply voltage from the outside, and the charging circuit performs the circuit operation with the power supply voltage.
When a user uses the external power supply circuit which should normally be used, the unexpected overvoltage is never applied to the charging circuit. On the other hand, when the user uses an external power supply incompatible with the electronic equipment, the extremely high overvoltage exceeding the breakdown voltage is possibly applied to the charging apparatus.